The government's index of leading economic indicators rose 1.2 percent in November, adding to the evidence the economy still is growing quite strongly, the Commerce Department reported yesterday.

President-elect Reagan said yesterday he does not expect to change his economic policy plans as a result of yesterday's report. He said he does not think the figures "mark a trend." Many economists both in and out of the government expect the economy to slow down soon.

"We don't see a renewed recession," said Commerce Department economist Ted Torda, "but we do expect the pace of economic growth to slow down" because of high interest rates and a sharp rise in payroll taxes that starts today. Torda said that yesterday's figures "are going to surprise some people" because of their strength, and indeed had increased more than the department expected.

The November rise followed an increase of 0.7 percent in October and was the sixth consecutive month when the leading index has gone up. "By themselves the October and November rises are relatively large," Torda said. It is usual for the rises in the leading indicators to slow down after the first upturn in the cycle.

But when asked whether he expected a further rise in the December numbers, Torda replied, "Frankly, no." He said that looking down the list of the indicators which are supposed to give a guide to the future course of the economy it was hard to see much strength in any of them this month.

Record-high interest rates are expected to brake the economy, particularly slowing demand for housing and autos. But there are signs that interest rates now have peaked. Another major bank, Irving Trust Co., yesterday lowered its prime lending rate to 20 1/2 percent from the record 21 1/2 percent still held by most banks.

A spokesman for Irving Trust said the reduction was spurred by recent declines in the bank's cost of acquiring funds. But he added that the move should not be interpreted as indicating the bank believes rates will continue to fall. Many analysts expect interest rates to remain fairly high in the coming year even if they drop from their present levels soon.

A surge in contracts and orders for new plant and equipment was the biggest factor pushing up the leading index in November, Torda said. This indicator rose by 17 percent in the month, after allowing for inflation. However the dramatic increase may well be followed by a bounce back down in December.

Other elements in the November rise in the overall index were a buoyant stock market in the immediate aftermath of the Reagan victory; a surprising rise in building permits, which Torda said probably will be followed by sharp falls because of high mortgage interest rates; rises in the average work week; and a decline in the number of layoffs.

Two of the 10 items making up the leading index declined: the real money supply and new orders after allowing for inflation.

Meanwhile, the Agriculture Department yesterday reported a drop of 1.1 percent in farm prices in December from November levels. In November farm prices had risen by 1.9 percent. December prices were still 9.2 percent higher than a year earlier.